Enteroendocrine cells play a key role in nutrient digestion and absorption, and are essential for normal life. The enteroendocrine L cell is of particular interest because it has the capacity to sense contents of the gut lumen and, in response, secretes several hormones. Two of those hormones are at the forefront of diabetes and obe- sity research: 1) the insulin-stimulating glucagon-like peptide 1 (GLP-1); and, 2) the satiety-inducing peptide tyrosine tyrosine (PYY). To study L cells, the applicant's laboratory has developed a transgenic mouse model in which the PYY promoter drives the expression of enhanced green fluorescent protein (GFP). Using the PYY- GFP model along with confocal laser scanning microscopy (LSCM), the applicant and his team have characterized the existence in L cells of a long (more than 50 5m) basal cytoplasmic process that resembles an axon. The long-term goal of the applicant is to understand how enteroendocrine cells, in particular the L cell, ex- change molecular information with enteric neurons to induce satiety and regulate appetite. The objective of this proposal is to determine the importance of the L cell basal process in the function of L cells, by assessing its role in PYY secretion and by characterizing the factors that regulate its growth and maintenance. The central hypothesis is that the L cell basal process is nurtured by specific signals derived from enteric glia, and functions as a pseudo-axon. The hypothesis has been formulated based on preliminary data developed by the applicant that show that L cells have an intimate relationship with elements of the enteric nervous system. The rationale is that by defining the function and regulation of basal processes new approaches can be developed to modulate hormone secretion from L cells. The central hypothesis will be tested in two specific aims: Aim 1 is to determine the effects of neurotrophic factors on the axon-like extension in L cells. The applicant will examine axonal process elongation in cultured primary L cells and in an intestinal organoid model that contains L cells. Aim 2 is to characterize the neurofilaments in L cell basal processes and define their role in PYY secretion. The applicant will use RNAi techniques to study in vitro and in vivo the neurofilaments found in L cells. Because PYY is a major satiety signal, the physical connection between the L cell and enteric neurons is ex- pected to carry important consequences for the regulation of appetite. The proposed project will provide the applicant with the required training to become an independent investigator in the field of gastrointestinal endocrinology.